JPH0445448B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The production of polycrystalline silicon for use in semiconductor devices is commonly accomplished by hydrogen reduction of various chlorosilanes at elevated temperatures. Due to the strict purity limits for semiconductor silicon, the starting material should be of the highest possible purity. Boron impurities in semiconductor silicon are generally the most troublesome. This is because boron in silicon has a partition coefficient close to 1 and therefore cannot normally be removed from silicon metal by conventional zone refining methods. Unfortunately, the boron-containing impurities are also difficult to remove from chlorosilanes.

There have been numerous attempts in the industry to purify chlorosilanes to produce better quality semiconductor silicon. These purification techniques include, for example, repeated distillation, water treatment (e.g.
(see US Pat. No. 3,540,861, issued Nov. 17) and the use of adsorbents to remove boron-containing impurities, and possibly also other impurities. U.S. Patent No. 4112057 (September 5, 1978)
(Published in Japan) treats a liquid chlorosilane with an effective amount of a hydrated metal oxide or hydrated silicate containing 3 to 8% water by weight, and then the treated chlorosilane is heated below its boiling point at atmospheric pressure. It is taught that chlorosilanes contaminated with boron-containing impurities can be purified by distillation at temperatures about 3-15°C higher. The above U.S. patent no.
For the adsorbent of '4112057, it is necessary to periodically add water to maintain the water content in the range of 3-8%. Although silica is used as an adsorbent in this method, only liquid phase adsorption is disclosed in the above-mentioned US Pat. No. 4,112,057.

U.S. Pat. No. 3,071,444 (issued January 1, 1963) teaches that chlorosilanes can be purified by passing liquid chlorosilane through layers of various adsorbents, including silica. However, for silica to be effective, it was necessary to preactivate the material in air for an extended period of time at temperatures of about 270 DEG C. or higher. Additionally, long contact times between chlorosilane and adsorbent were employed. Said U.S. Patent No. 3071444
No. 5,922,301 does not disclose a method for purifying chlorosilane by passing the gas phase chlorosilane through any of the disclosed adsorbents.

Chlorosilanes (U.S. Pat. No. 2,877,097, issued March 10, 1959) and organochlorosilanes (U.S. Pat. No. 3,414,603, issued December 3, 1968).
Adsorption techniques using ion exchange resins have also been used for the purification of ion exchange resins. Both patents teach that the adsorption of impurities (including boron) from chlorosilane by passage through an ion exchange resin was independent of the state of the chlorosilane (liquid or gas phase). In other words, the above U.S. Pat. No. 2,877,097;
No. 3414603 each specification describes various chlorosilanes,
It teaches that liquid phase and gas phase adsorption of impurities using ion exchange resins are equivalent methods.

One object of the present invention is to provide a method by which chlorosilane can be purified more easily. Another object is to provide a method by which boron-containing impurities can be more easily removed from chlorosilane. Yet another object is to provide a method for producing trichlorosilane which is suitable for use in the production of semiconductor silicon and is substantially free of boron-containing impurities. Still other objects of the invention will be apparent to those skilled in the art upon consideration of this specification.

The present invention relates to a method for purifying a chlorosilane selected from the group consisting of monochlorosilane, dichlorosilane, trichlorosilane and tetrachlorosilane, wherein said chlorosilane is contaminated with boron-containing impurities, and said method comprises: (A) chlorosilane is passed through a silica bed in the gas phase,
The silica bed is at a temperature 3° C. or more higher than the boiling point of the chlorosilane at the operating pressure of this step (A),
and (B) collecting purified chlorosilane having a significantly reduced amount of boron-containing impurities.

The present invention relates to a method for removing boron-containing impurities from various chlorosilanes, thereby making them more suitable for the production of semiconductor silicon by various known reduction techniques, particularly hydrogen reduction techniques. Chlorosilanes suitable for purification by the method of the invention include those having the general formula: Cl _o SiH _4-o , where n is 1, 2, 3 or 4. More specifically, these chlorosilanes include monochlorosilane, dichlorosilane, trichlorosilane, and tetrachlorosilane. Dichlorosilane and trichlorosilane are the preferred species for the practice of this invention. Trichlorosilane produced in the so-called "direct process" reaction of silicon metal with hydrogen chloride is a particularly preferred species for the practice of this invention.

The method of the present invention involves converting vaporized chlorosilane into a silica bed.
This is preferably done by passing the chlorosilane through a fixed bed of silica in an adsorption column or tower where significant amounts of boron-containing impurities are removed from the chlorosilane and then collecting the purified chlorosilane. For the process of the invention to be advantageous, the chlorosilane must be in the gas phase during contact with the silica. Advantages of the process of the present invention over prior art purification methods include, among others, improved ability of silica to remove boron impurities, improved efficiency for boron removal, and improved efficiency between silica and chlorosilane for boron removal. Includes reduced contact time. These advantages will be explained in more detail in Examples below.

The temperature of the silica bed must be such that the chlorosilane remains in the gas phase. Therefore,
The temperature of the silica gel bed should be about 3° C. or more above the boiling point of the chlorosilane under the operating pressure of the silica gel bed. It has been found that the adsorption capacity and efficiency of a silica bed decreases when the temperature of the silica bed rises above about 85°C. Therefore, it is preferred to keep the temperature of the silica bed below about 85°C to achieve maximum purification. When trichlorosilane is purified by the procedure of the present invention, it is particularly preferred that the adsorption bed temperature be between about 50 and 75°C.

The pressure in the adsorption column is not critical. However, as will be understood by those skilled in the art, if operation within the preferred temperature range is desired, the pressure should not be so great that the above temperature limits cannot be accommodated. In other words, if it is desired to operate the silica bed in a preferred temperature range of 3° C. above the boiling point of the chlorosilane, but below about 85° C., the adsorption column pressure should be equal to or below the boiling point of the chlorosilane to be purified. The pressure should be less than that required to raise the temperature to a temperature of about 82°C. If not so desired, the pressure can be subatmospheric or superatmospheric.

Forms of silica useful in the present invention include silica gel, silica powder, fumed silica, amorphous silica, and precipitated silica.
The silica used in the practice of this invention was as received from the manufacturer or supplier. The free water content of the silica is determined in a forced air convection oven.
Measured by weight loss over 3 hours at 105°C. The silica tested had a free water content of about 0-6% by weight. The total hydroxyl content of the silica was determined by titration with lithium aluminum di-n-butylamide. This titration method was developed by AL Smith (Ed), New York City, USA.
John Wiley and Sons
Sons (1974), "Amalysis of Silicones", pp. 139-142, as detailed below.

``Total hydroxyl titration method'' KER (Karl Fitscher reagent) titration has been used to quantify silanol + water. When many monomers and unhindered silanol groups are reacted with methanol and titrated or contacted with excess KFR,
One mole of water is produced per mole of SiOH.

≡SiOH+MeOH→≡SiOMe+H ₂ O (1) H ₂ O+I ₂ +SO ₂ +MeOH→2Hl+MeHSO ₄ (2) If too much KFR is added, back titration with standard water-methanol is used. Many silanol groups react incompletely or slowly in alcoholic solvents due to problems such as steric hindrance and limited solubility. This method applies best to low molecular weight silanols or monomers. Using a slightly modified method, a fast reaction rate with water can be achieved with silanol-
It can be distinguished from the methanol reaction. Cyclotrisiloxane reacts rapidly with methanol to form water and cause significant interference. However, siloxanes of other structures do not seem to cause any interference even if they react sufficiently.

Titration of total hydroxyls (HOH, SiOH, COH) in silicone compounds using lithium aluminum di-n-butylamide has been reported.

LiAl(NBu ₂ ) ₄ +4ROH→4Bu ₂ NH+LiOR+Al(O
R) ₂ (3) Mercaptan (SH) is also quantitatively titrated.
Improvements in the design of reagent reservoirs and barrels have produced reagents that are practical and rather easily degraded by routine use. There is almost no interference,
Bonds of SiH, SiOR, and SiOSi are also unaffected. Acetoxy compounds interfere with indicator reactions. Since water reacts stoichiometrically, its recovery is approximately 75% under these titration conditions. If the ratio of water to SiOH changes relatively significantly, the total hydroxyl number will become somewhat less skewed.

The main advantages of this all-hydroxyl method are its rapidity,
Both PPM and percentage levels of hydroxyl are accurate and generally applicable. Once the titration vessel has been titrated with the amide reagent, standardization has been performed, and it has "dryed up," the sample will be analyzed in 1-2 minutes. The normality of the reagent is such that samples containing more than 10% hydroxyl can be reproducibly titrated from 1 PPM.
It is allowed to vary between 0.02 and 0.5.
Diluted titration standards are best standardized using solid standards. For example, 2-naphthol is used, and 5 to 10 mg of it can be easily weighed.
A variety of titration solvents are used. Tetrahydrofuran (THF), THF-pyridine or toluene dissolve silicone fluids and resins fairly well.
However, very high viscosity fluids and rubbers do not work well. Some produce gels or precipitates after one or more titrations. This problem is minimized by frequently changing the solvent.

Reagents Ethylene glycol, dimethyl ether (Ansul 121, Monoglyme) Di-n-butylamine, Eastman 1260
4-Phenylazodiphenylamine, Eastman 1714 Dissolve in benzene to make a 0.1% solution.

Lithium aluminum hydride 10 mesh, 95
% 2-Naphthol, Eastman 171 Dry in a dessicator using barium oxide.

A stock reagent of lithium aluminum dibutylamide titration standard solution 0.5N is prepared as follows. Monoglyme, dibutylamine and
The THF is dried by slowly flowing through a 3-ft molecular sieve (5A, 8 x 12 mesh beads) tower.

Add 3600ml of Monoglyme to the three-necked flask from Step 5. Add a few drops of indicator and stir with a magnetic stirrer to begin _N2 substitution. Add a small amount of _LiAlH4 until the red indicator color persists (a slight excess of _LiAlH4 is present).

Add 25g of LiAlH ₄ and stir until dissolved. Attach a water condenser to the flask and gently reflux the mixture for 20 minutes. Slowly add 500 ml of dibutylamine through the dropping funnel and reflux for 10 minutes. Continue the _N2 purge for 24 hours until the solid becomes clear.
Transfer the clear liquid through polyethylene tubing to a 125 ml vial by applying slight N ₂ pressure to the flask.
Seal each vial with a Teflon-rubber disc and aluminum wire using a crimper. (The reagent can be stored stably in this state from June to December.)
Using the dried monoglyme in the 125ml small bottle~
The desired amide titration standard solution can be prepared by diluting the 0.5N stock reagent. Seal using butyl rubber septum, aluminum stopper and crimper.

〔Device〕

2 ml barrel (see Figure 1) This barrel has been modified to include a custom 2 ml self-zeroing barrel with a 2 mm inner diameter glass Luer joint, as well as a dispensing and refilling tip to which a hypodermic needle can be connected. I took over the department.

Hypodermic injection needle 2.5cm and 12.7cm length Attached small bottle 125ml Piercing Chemical Company Crimper,
With aluminum stopper, Teflon-rubber laminated disc and butyl rubber diaphragm.

〔operation〕

Connect a 2.5 cm needle to the dispensing tip of the bottle and a 12.7 cm needle to the fill port. A 12.7 cm needle is inserted through the septum of the titration standard vial.
Pressurize the reagent bottle with a 2.5 cm needle connected to a dry nitrogen source to fill the bottle. 20-25 ml dry THF (or other solvent like toluene, THF-pyridine)
into the attached 50 ml vial and cover with a butyl rubber septum. (Do not plug.) Insert the needle at the tip of the bottle, add the indicator, move the magnetic stirrer, and continue titrating for 30 seconds or longer until the indicator changes from yellow to the end point of reddish-purple. The amide reagent is standardized daily with weighed 2-naphthol in aluminum dishes or iodine flasks. Or 0.1N
Dry amyl alcohol is used to standardize stronger reagents.

Add the liquid or solid sample in the same manner and titrate for 30 seconds until the end point is stable. Usually 10 or more samples can be analyzed using the same titration solvent until the reaction products precipitate. If gelation occurs, the solvent must be changed between titrations.

The formula for calculating the percentage of total hydroxyls is:

%OH = (ml) (N amide) (0.017) (100) / sample weight (g) (4) If all hydroxyls are assumed to be water, the calculation formula is as follows.

%H ₂ O = (ml) (N amide) (0.017) (100) / [sample weight (g)] (0.75) ^* (1.89) ^** (5) * 75% recovery when the standard is alcohol Factors to correct for differences in ** coefficients for 2(OH)/H ₂ O These procedures determine the water content and ≡ on silica
Both SiOH surface groups are measured. The silica was not dried before measuring total hydroxyls. Lithium aluminum di-n-butylamide titration on the test sample determined that the total hydroxyl content was between ca.
It was found that the content was within the range of 3% (by weight). The performance of gas phase adsorption methods for boron removal from chlorosilanes does appear to vary with the total hydroxyl content of the silica. With the exception of silica, which contains very small amounts of total hydroxyls, the variation in performance is small. It is anticipated that silica having less than about 0.25 weight percent total hydroxyls will be unsuitable for use in the present invention. It is preferred that the silica contain at least 1.0% total hydroxyls for best performance. As known to those skilled in the art, there are maximum limits to the free water content of the silica used in the present invention. This maximum limit arises from the possibility of hydrolysis of the chlorosilane. Excessive hydrolysis caused by excess water in the silica can clog the adsorption column, thereby restricting vapor flow through the column and causing excessive pressure drop across the column. . To obtain maximum utility of the present invention, the use of "wet" silica as described above should be avoided.
To date, no column clogging has been observed in any of the experiments conducted using the gas phase adsorption method. However, it is determined that the free water content should be about 10% by weight or less to minimize the potential for column clogging problems.

The particle size and particle size distribution of the silica can also vary. The best silica tested to date is
Approximately 35 to 65 of particles, with a particle size of 28 to 200 mesh
It has a particle size distribution such that 65% remains on the sieve. As the particle size decreases, the pressure drop across the fixed bed of silica increases. Silica bed columns with high pressure drops do work in the practice of this invention, but the high capital investment due to the larger pumps and other equipment required to overcome the pressure drops Requires. As the particle size of silica increases, it is expected that the efficiency and capacity for purification of chlorosilanes will decrease. For example, silica having a particle size of 12 to 28 meshes exhibits poorer results than materials of 28 to 200 meshes. The 12-28 mesh substance has a silica content of 28-200 mesh.
Capacity of 1500~ _2000HSiCl3 / _gSiO2 and boron approx. 90
% removal, it has a capacity of about 500 gHSiCl ₃ /gSiO ₂ and removes only about 63% of the boron. However, the performance of larger particle size materials is still superior for gas phase adsorption systems compared to liquid phase adsorption systems.

All silicas tested had large surface areas. It is recognized that silica having a surface area greater than 100 m ² /g is suitable for the practice of the present invention. However, silica with a surface area greater than 500 m ² /g is preferred.

The process of the present invention is carried out by passing vaporous chlorosilane through silica and then collecting the purified chlorosilane effluent. Passing the chlorosilane through the silica produces an effluent containing significantly reduced amounts of boron-containing impurities compared to the raw chlorosilane feed. The meaning of "significantly reduced amount" is related to the boron content of the chlorosilane feedstock. Although boron is contained in chlorosilane in the form of a compound rather than in an atomic state, the "boron content" described herein is a value obtained by converting the amount of the boron compound contained into the amount of boron atoms. If the chlorosilane feedstock contains more than about 500 ppba (parts per billion atoms) of boron, a "significantly reduced amount" means that at least 75% of the boron-containing impurities are removed from the chlorosilane feedstock. be understood. Chlorosilane raw material
When containing between 50 and 500 ppba of boron, "significantly reduced amount" is understood to mean that at least 50% of the boron-containing impurities are removed from the chlorosilane feedstock. When a chlorosilane feed contains 50 ppba or less of boron, "significantly reduced amount" is understood to mean that at least 25% of the boron-containing impurities are removed from the chlorosilane feed by practicing the present invention.
In most cases, more than 90% of the boron-containing impurities are removed from the crude chlorosilane feed by passing the steam through a suitable silica adsorption column. The purified chlorosilane effluent stream is typically
Contains less than 100 ppba of boron, often less than 50 ppba. The capacity of most of the silicas tested in the method of the invention was in the range of 1500-2000 _gHSiCl3 / _gSiO2 . The boron content in the effluent increases as the capacity of the silica column is approached. When the boron content of the effluent reaches approximately 150 ppba boron,
The column is said to have reached a "breakthrough" level of boron and is considered nearly depleted. If more chlorosilane is passed through the column after the leakage point is reached, the boron content of the effluent will rapidly rise to a high level, approaching the level of boron in the feed chlorosilane. That is, if
If purification of chlorosilanes containing less than 150 ppba of boron is intended, this definition of "leakage" does not apply.

The process of the present invention can be used alone to remove boron-containing impurities from chlorosilanes. The process can be carried out continuously, semi-continuously or batchwise as desired. This purification method can also be used in combination with other chlorosilane purification methods. For example, the method of the present invention can be used with prior art iterative distillation procedures. A gas phase adsorption column can be placed before the first distillation column, between distillation columns, or after the last column. The gas phase adsorption technique can also be used in conjunction with water treatment, with or without additional distillation. More than one silica column can be used in such purification methods. Indeed, one of the advantages of the process of the invention is that it can be easily incorporated into current chlorosilane purification processes.
One advantage of combining the method of the present invention with other methods to purify chlorosilanes is that process upsets in any part of the system are directly utilized to produce polycrystalline silicon. This means that there is little possibility of resulting in the formation of unsuitable chlorosilanes. Such upsets may occur in gas phase adsorption processes (eg early boron leakage) or in other parts of the combined process. Another advantage of combining the purification method of the present invention with other purification methods is an improvement in the quality of the purified chlorosilane. This combined purification system can also be operated in continuous, semi-continuous or batch mode.

The following examples are illustrative of the invention and are not intended to limit it. Examples 1-5 and Comparative Examples 1-3 were conducted in small laboratory scale equipment. Examples 6 and 7 each illustrate the practice of the invention using larger scale equipment. In each example, the boron content of the effluent chlorosilane was determined using the analytical procedure described below. The analyzer is entirely made of Teflon.
was configured to minimize sample contamination and sample exposure to air. The analyzer consisted of a sample container for the effluent to be tested, which had an argon inlet tube and a diptube extending very close to the bottom of the sample container. The top of the dip tube is such that when a liquid chlorosilane sample is placed in the sample container and the system is pressurized with argon, the chlorosilane flows up the dip tube, enters the top of the adsorption packed column, and passes through the packed column. , and then connected to the receiver so as to exit toward the receiver. The adsorption column contained amorphous silica (Cabosil S-17, Cabot Corporation, Boston, Mass., USA), particularly selected for its low levels of boron. . The silica is prepared by first adding argon to Ultrex.
48% HF aqueous solution [manufactured by JT Baker, Phillipsburg, New York, USA] approx.
Hydrofluoric acid treatment was performed by passing over 100 mg of silica and then through a column (containing approximately 75 mg of silica). 75-150 argon containing HF vapor
Passed through silica at a rate of ml/min for 45 minutes.
The fluorinated column contained approximately 0.5% fluorine after purging with argon. For practical analysis, the effluent chlorosilane was placed in a sample container.
The sample container is pressurized with welding grade argon and passed through a fluorinated amorphous silica column to provide a liquid elution rate of about 50 ml per hour of chlorosilane. After elution was complete, argon was flowed through the analytical column at a rate of about 75-150 ml/hour for about 45 minutes. The concentration of boron adsorbed on the analytical column was then measured by atomic emission spectrography. In fact, the analytical column was used to concentrate boron-containing impurities present in the effluent. Therefore, the detection limit for boron is largely related to the amount of effluent passing through the analytical column. The amount of boron in the effluent is determined by the following formula: Boron concentration in the column (ppm) x Column weight x F/Sample weight [In the formula, "Boron concentration in the column (ppm)" is the concentration of boron in the analysis. is the concentration of boron adsorbed on the column (in parts per million), "column weight" is the weight of the material in the analytical column, and "sample weight" is the weight of the chlorosilane effluent passing through the analytical column. can be,
"F" is a factor for converting boron ppm (weight) to the desired ppba]. For trichlorosilane, F is approximately equal to (1000(135.5)/10.8, where 135.5 is the molecular weight of trichlorosilane and 10.8 is the atomic weight of boron.

In Comparative Examples 1 to 3, the analytical column was actually incorporated directly into the experimental system. In these examples, first the liquid chlorosilane is passed through the test column;
It was then passed directly through an analytical column. The analytical column was removed periodically to determine the boron content in the effluent.

Example 1 This example demonstrates the gas phase adsorption of boron-containing impurities from trichlorosilane. The trichlorosilane used contained approximately 7900 ppba of boron. The silica used was from Baltimore City, Maryland, USA.
WR Grace and Company (Grace
and Davidson Chemical Div. Davidson grade 12 silica has a free water content of approximately 4%, a hydroxyl content of approximately 3%, and a particle size of 28 to 288.
Metsuyu, bulk density approx. 73Kg/ ^m3 , surface area approx. 720~
760 m ² /g, and a pore volume of about 0.4 cc/g.
The particle size distribution of the silica was as follows: 6% remained on the 28-mesh sieve, 50% remained on the 65-mesh sieve, 85% remained on the 150-mesh sieve, and 200% remained on the 150-mesh sieve. 94% remained on the mesh sieve. All data for the silica, other than moisture content, were provided by the vendor.

The equipment used was constructed of stainless steel except for the collection bottle, which was made of Teflon. Chlorosilane contaminated with boron impurities was charged into a raw material tank under a nitrogen atmosphere, and by changing the nitrogen pressure above the chlorosilane in the raw material tank,
The flow rate of chlorosilane through the system was controlled. Upon exiting the feed tank, the liquid chlorosilane passed through a rotameter where the flow rate of liquid trichlorosilane was monitored. After the rotameter, the liquid chlorosilane enters a steam-heated coil evaporator under a pressure of about 4.5 Kg. Sufficient steam is used in the evaporator to ensure that all chlorosilane is evaporated. The chlorosilane vapor is then passed through a test column containing silica adsorbent. The conduit between the evaporator and the test column and the test column itself are heat traced and carefully maintained at a specified temperature above the boiling point of the chlorosilane. After passing through the test column, the chlorosilane vapor is condensed at dry ice temperature and collected in a Teflon container that is also cooled with dry ice. The harvest was then analyzed for boron content. Typically several chlorosilane samples are taken throughout the course of the experiment;
For the evaluation of a given silica, it was analyzed for boron.

In a blank run, trichlorosilane containing approximately 7900 ppba of boron was passed through the system in the absence of adsorbent material in the test column. The temperature of the test column was 50°C. The collected trichlorosilane was found to contain approximately 3600 ppba of boron.

Davidson grade 12 silica (456 g) was placed in the test column. The fixed bed of silica was approximately 2.5 cm thick. Trichlorosilane containing about 7900 ppba of boron was fed to the evaporator at a liquid flow rate of about 1.34 g/min. Trichlorosilane vapor approx.
It passed through the test column at a speed of 8.2 m/min with a residence time of approximately 0.2 seconds. The temperature of the test column is
It was 50℃. The pressure drop across the fixed bed was approximately 0 Kg/cm ² . A total of 482 g of trichlorosilane vapor was passed through the fixed bed of silica. No boron was detected in any of the effluents (detection limit is based on the size of the HSiCl ₃ sample, approximately
60ppba). The capacity of this silica column is
It was larger than 1060gHSiCl ₃ /gSiO ₂ . More than 99.2% of boron was removed by passing through the column. No "leakage" of boron was observed.

A comparison of the blank run and the run containing grade 12 silica shows that the reduction in boron impurities described in this and other examples is due to the silica and not simply to chlorosilane evaporation and condensation.

Example 2 This example also demonstrates the gas phase adsorption of boron impurities from chlorosilanes. The apparatus described in Example 1 above was used. The trichlorosilane feedstock (containing approximately 7900 ppba of boron) and silica (Davidson grade 12) used were the same as those used in Example 1. Approximately 785 mg of silica was placed in the test column.
Then add trichlorosilane (379g) at a liquid flow rate of 0.36
cc/min through the system, resulting in a vapor residence time of approximately 0.9 seconds in the fixed bed of silica. The temperature of the fixed bed was 50°C. The pressure drop across the fixed bed was approximately 0 Kg/cm ² . No boron was detected in the effluent (detection limit approximately 30 ppba). The ability of silica to purify trichlorosilane is
It was larger than 506gHSiCl ₃ /gSiO ₂ , and the boron removal rate was 99.6% or more. No "leakage" of boron was observed.

Example 3 Trichlorosilane containing 3500 ppba of boron was prepared to Davidson grade at 50° C. using a method and materials similar to those described in Example 1 above.
Purification was performed by gas phase adsorption using No. 12 silica (free water content approximately 4%, hydroxyl content approximately 3%). The test column was loaded with 466 mg of _SiO2 . The residence time of vapor in the column was 0.2 seconds.
After a total of 944 g of HSiCl ₃ had been purified, the effluent still contained undetectable amounts (<50 ppba) of boron. The capacity of this silica is 2025gHSiCl ₃ /
gSiO ₂ or higher, and the boron removal rate was 98.6% or higher.

Comparative Example 1 This example demonstrates liquid phase adsorption of boron impurities from trichlorosilane using Davidson Grade 12 silica. The free water content of the silica was about 4% and the hydroxyl content was about 3%. The device was constructed of stainless steel and Teflon. Chlorosilane was charged into a raw material tank connected to a nitrogen conduit. The flow rate of chlorosilane was controlled by varying the nitrogen pressure. Three Teflon test columns (inner diameter
0.64 cm, length 5.1 cm). The test column contained the silica to be tested. After passing through the test column, the chlorosilane flowed through the analytical column and then into the purified product collection vessel. The analytical column contained approximately 100 mg of HF-treated amorphous silica Cabosil S-17. The analytical column was frequently removed and the effluent was analyzed for boron content. A new analytical column was used after each boron analysis.

The trichlorosilane feed contained 7650 ppba of boron. Each test column contained approximately 1 g of silica for a total of 3.19 g. The system was operated at ambient temperature (approximately 22°C). After passing 662 g of HSiCl ₃ through the system, the boron content of the effluent was still below the detection limit (approximately 90 ppba). By passing an additional 189 g of HSiCl ₃ , the effluent contained 126 ppba of boron. An additional 214 g of HSiCl ₃ reduced the effluent to
Boron levels reached 740ppba. Therefore, the silica reached the "leakage" level on the two occasions mentioned above.
Passage of HSiCl ₃ (passed liquid amount = 662g + 189g = 851g)
and the above three passes of HSiCl ₃ (passed liquid amount = 662g +
189g + 214g = 1065g). Therefore, the "leakage" capacity of the silica is the amount of these passing liquids divided by the total amount of silica, 3.19 g, or 851/3.19≒
_270gHSiCl3 / _gSiO2 and 1065/3.19≒
It is between 330gHSiCl ₃ /gSiO ₂ . Also, "leakage"
More than 98.8% of boron was removed in the previous step.

A comparison of Examples 1, 2, and 3 with Comparative Example 1 clearly shows the excellent results obtained by gas phase adsorption.

Example 4 This example illustrates the purification of a relatively pure sample of trichlorosilane using gas phase adsorption on a fixed bed of silica. The equipment used was that described in Example 1 above. The starting trichlorosilane contained 410 ppba of boron. The silica (466 g) was Davidson grade 12 and had a free water content of about 4% and a hydroxyl content of about 3%.
The column temperature was 50°C. The vapor velocity in the silica column was approximately 8.2 m/min. The pressure drop across the fixed bed was approximately 0 Kg/cm ² . 590g
The effluent had undetectable boron levels even after 300 ml of HSiCl ₃ had passed through the column. The detection limit was approximately 40 ppba. The ability of silica to remove boron from relatively pure trichlorosilane was greater than 1266 gHSiCl ₃ /gSiO ₂ . More than 90% of boron impurities were removed.

Comparative Example 2 This example illustrates an attempt to purify a relatively pure sample of trichlorosilane using liquid phase adsorption on a fixed bed of silica. The trichlorosilane sample contained 455 ppba of boron. The silica (2.99 g) was Davidson grade 12 and had a free water content of about 4% and a hydroxyl content of about 3%.
The flow rate of trichlorosilane through the system is approximately 1 c.c./
It was hot in minutes. The temperature of the system was approximately 22°C. 402g
of HSiCl ₃ was passed through the fixed bed before the first boron measurement was performed. The effluent contained 187 ppba of boron. Therefore, relatively pure HSiCl ₃
The ability of liquid phase adsorption method for the purification of
It was less than 134gHSiCl ₃ /gSiO ₂ , and the boron removal rate was only about 60%.

By comparing Example 4 and Comparative Example 2, the superiority of the gas phase adsorption method is clearly demonstrated.

Example 5 This example demonstrates the purification of trichlorosilane by a gas phase adsorption procedure using different silicas as adsorbents. The silica used has a surface area of approximately 190 m ² /g,
Sipernat 22, Teter, New Jersey, USA, with an average particle size of 18 microns, a bulk density of approximately 200 g/2, a hydroxyl content of approximately 3%, and a free water content of approximately 6% as shipped from the manufacturer.
It was made by Degussa, Boro City.
The equipment and procedures described in Example 1 above were used.
Silica (300 mg) was charged to the test column. The temperature of the test column was 50°C. The crude trichlorosilane contained 5700 ppba of boron. The vapor flow through the test column was 4.0 m/min. The residence time of vapor in the column was approximately 0.8 seconds. After 299 g of HSiCl ₃ had passed through the column, the resulting effluent contained 56 ppba of boron. When an additional 170 g of HSiCl ₃ vapor was passed through the column, the effluent contained 49 ppba of boron. The pressure drop across the column (about 5 cm long) was about 18.5 Kg/cm ² /m. The capacity of the adsorbent is more than 1560gHSiCl ₃ /gSiO ₂ , which is approximately
99.1% removed.

In a similar experiment, except that the vapor residence time in the silica column was increased to approximately 1.3 seconds,
Degustation sash per nuts under the same experimental conditions as above.
A fresh sample of 22 silica (265 mg) was used.
Unpurified trichlorosilane (boron
After passing approximately 290 g (5700 ppba), the effluent contained no detectable amounts of boron (less than 30 ppba). The capacity of this silica for boron removal was greater than 1094 gHSCl ₃ /gSiO ₂ and about 99.5% of boron was removed. The pressure drop across the column was approximately 6.9 Kg/cm ² /m.

Comparative Example 3 This example demonstrates the purification of trichlorosilane by liquid phase adsorption using Degutsu Sacipelnut 22 silica. The procedure and equipment used were the same as described in Comparative Example 1 above, and the silica (245 g) was as described in Example 5 above. Liquid trichlorosilane (8024 ppba boron) was passed through the silica column at a rate of approximately 1.2 cc/min and at ambient temperature (22°C). After passing 166g of trichlorosilane,
The effluent contained 517 ppba of boron. The capacity of the silica column was 678 gHSiCl ₃ /gSiO ₂ or less, and the boron removal rate was about 94%.

By examining Example 5 and Comparative Example 3, it is clear that gas phase adsorption is superior to liquid phase adsorption using Degutsusa Cipernat 22 silica as an adsorbent.

Examples 6 and 7 below are “mini-
The implementation of the invention on a "plant" scale is illustrated. Liquid chlorosilane was fed into the system through a rotameter. The liquid trichlorosilane was passed from the rotameter into the evaporator. The carbon steel evaporator consisted of a jacket approximately 10.2 m inside diameter and approximately 0.61 m long containing approximately 7.9 m of 0.64 cm stainless steel conduit in the form of a coil. 13.6 Kg of steam was passed through the coiled conduit so that the evaporator operated at approximately 120°C. The trichlorosilane vapor was then transported through a heat traced (approximately 70°C) carbon steel conduit to a fixed bed adsorption unit. The fixed bed adsorption unit was made of carbon steel and had a length of 30 cm and a diameter of 6.3 cm. 350
Three Watt tubular heaters were attached to the outside of the adsorption unit. Silica was contained in the adsorption unit by a 325 mesh stainless steel gauze mounted on a support plate. Vaporous trichlorosilane was passed from the adsorption unit into a sample cooler cooled by service water (approximately 25°C). The condensed trichlorosilane was then collected.

Example 6 This example demonstrates the effect of temperature on an adsorption process using the "miniature device" system described above. Davidson grade 12 silica containing 2% free water and 3.0% total hydroxyls was used. Silica in each experiment
It was loaded into a 1.0Kg adsorption unit. The crude silica contained approximately 8000 ppba of boron. The flow rate of trichlorosilane through the system was 15.9 Kg/hr. The pressure drop across the fixed bed was in the range of 0.15-0.3 Kg/cm ² in all experiments. In Experiment 1, the adsorption unit had an average temperature of 85°C and a temperature range of 73-99°C. In Experiment 2, the average temperature of the silica gel bed was 74°C, and the temperature range was 63-85°C. In Experiment 3, the average temperature was 62°C and the temperature range was 59-64°C.
In experiment 1, after passing 400 g HSiCl ₃ /g SiO ₂ , the effluent contained 380 ppba boron. Therefore, at an average temperature of 85°C, the fixed bed capacity is
It was 400gHSiCl ₃ /gSiO ₂ or less, and the boron removal rate was about 95%. In experiment 2
After passing through _400gHSiCl3 / _gSiO2 , the effluent contains boron
After passing through 1485gHSiCl ₃ /gSiO ₂ the effluent contains 85ppba boron, and
After _1950gHSiCl3 / _gSiO2 treatment, the effluent is boron
It contained 3000ppba. Therefore, at an operating temperature of about 74° C., the capacity of the silica for the purification of trichlorosilane is 1485 gHSiCl ₃ /g SiO _{2 .}
The boron removal rate was between 1950 gHSiCl ₃ /gSiO ₂ and about 98.9%. In experiment 3, after passing 500 g HSiCl ₃ /g SiO ₂ through the silica bed, the effluent contained less than 15 ppba boron and 1000 g HSiCl ₃ /g SiO 2 .
contains 15 ppba of boron after passing through _gSiO2 , and
The effluent contained only 60 ppba boron after 1530 g HSiCl ₃ /g SiO ₂ passed through the fixed bed of silica. Therefore, the capacity of silica for boron adsorption is _1500gHSiCl3 /at a temperature of about 62°C.
_gSiO2 or more, and about 99.2% of boron was removed.

Based on these results, the temperature of the fixed bed adsorption unit should be approximately 85°C for maximum capacity and efficiency in removing boron from trichlorosilane.
It is preferable to keep the temperature below the average temperature.

Example 7 This example demonstrates the quality of polycrystalline silicon that can be produced from trichlorosilane purified by the procedure of the present invention. The "miniature device" gas phase adsorption system was used. In experiment 1, Davidson magnitude
12 silica (used in Example 6)
1.0Kg was used in a fixed bed. The adsorption unit was maintained at an average temperature of 70°C (temperature range: 66-75°C). Trichlorosilane containing approximately 2700 ppba of boron was passed through the fixed bed at a flow rate of 15.9 Kg/hr. The pressure drop across the fixed bed was 0.3 Kg/cm ² . At a capacity of 1440 g HSiCl ₃ /g SiO ₂ the effluent contains detectable amounts of boron (the detection limit is
It contained 14 ppba). The polycrystalline silicon, prepared by conventional hydrogen reduction techniques using effluent trichlorosilane and without any additional purification, contained 0.20 ppba of boron. As is known to those skilled in the art, the polycrystalline silicon produced from the crude trichlorosilane used in Experiment 1 above and without any purification is likely to contain boron.
It will contain more than 1000 ppba.

In experiment 2, 1.0 kg of Davidson grade 12 silica from different lots was used in the adsorption unit. This silica contained no free water and approximately 1.0% of total hydroxyls. The crude chlorosilane contained approximately 100 ppba of boron and was passed through a fixed silica bed at a temperature between 63°C and 71°C and a flow rate of 15.4 Kg/hr. The capacity of this silica was greater than 1080 gHSiCl ₃ /gSiO ₂ and the boron in the effluent was less than 20 ppba. Polycrystalline silicon produced from this effluent (without further purification) contained 0.03 ppba of boron. boron
Polycrystalline silicon made from trichlorosilane with 100 ppba is estimated to contain 10-20 ppba or more of boron.

[Brief explanation of drawings]

Figure 1: Schematic diagram of a bulette for titration of active hydrogen with LiAl(NBu ₂ ) ₄ .

Claims (1)

[Claims] 1. A method for purifying chlorosilane selected from the group consisting of monochlorosilane, dichlorosilane, trichlorosilane, and tetrachlorosilane, and which is contaminated with boron-containing impurities, comprising:
(A) passing a chlorosilane in the gas phase through a bed of silica, wherein said silica bed is 3° C. below the boiling point of the chlorosilane under the operating pressure of step (A);
or higher temperature, and the silica bed comprises silica containing about 0-10% by weight free water and about 0.25% by weight or more total hydroxyls;
and then (B) collecting purified chlorosilane in which the amount of boron-containing impurities is significantly reduced. 2. The method of claim 1, wherein the total hydroxyl content of the silica is greater than or equal to about 1.0%, and the temperature of the silica bed is less than or equal to 85°C. 3. Claim 1 in which chlorosilane is passed through a fixed bed of silica in an adsorption column in the gas phase.
The method described in section. 4. The method of claim 3, wherein the temperature of the fixed bed of silica is between 50°C and 75°C. 5 The temperature of the silica bed is lower than 85°C, and the silica has a surface area of 500 m ² /g or more and a particle size distribution such that 35 to 65% of the particles remain on a 65 mesh sieve. The method described in item 1. 6 The temperature of the silica bed is between 50°C and 75°C,
Moreover, the total hydroxyl content of the silica is approximately 1.0.
% or more, the method according to claim 5.